This invention relates to electric discharge lasers and in particular to such lasers having chambers with long life electrodes.
The principal components of a prior art KrF excimer laser chambers are shown in FIG. 1. This chamber is a part of a laser system used as a light source for integrated circuit lithography. These components include a chamber housing 2. The housing contains two electrodes 84 and 83 each about 50 cm long and spaced apart by about 20 mm, a blower 4 for circulating a laser gas between the electrodes at velocities fast enough to clear (from a discharge region between the two electrodes) debris from one pulse prior to the next succeeding pulse at a pulse repetition rate in the range of 1000 Hz or greater, and a water cooled finned heat exchanger 6 for removing heat added to the laser gas by the fan and by electric discharges between the electrodes. The chamber may also include baffles and vanes for improving the aerodynamic geometry of the chamber. The laser gas is comprised of a mixture of about 0.1 percent fluorine, about 1.0 percent krypton and the rest neon. Each pulse is produced by applying a very high voltage potential across the electrodes with a pulse power supply 8 which causes a discharge between the electrodes lasting about 30 nanoseconds to produce a gain region about 20 mm high, 3 mm wide and 700 mm long. The discharge deposits about 2.5 J of energy into the gain region. As shown in FIG. 2, lasing is produced in a resonant cavity, defined by an output coupler 2A and a grating based line narrowing unit (called a line narrowing package or LNP, shown disproportionately large) 2B comprising a three prism beam expander, a tuning mirror and a grating disposed in a Littrow configuration. The energy of the output pulse 3 in this prior art KrF lithography laser is typically about 10 mJ.
There are many industrial applications of electric discharge lasers. One important use is as the light source for integrated circuit lithography machines. One such laser light source is the line narrowed KrF laser described above which produces a narrow band pulsed ultraviolet light beam at about 248 nm. These lasers typically operate in bursts of pulses at pulse rates of about 1000 to 4000 Hz. Each burst consists of a number of pulses, for example, about 80 pulses, one burst illuminating a single die section on a wafer with the bursts separated by down times of a fraction of a second while the lithography machine shifts the illumination between die sections. There is another down time of a few seconds when a new wafer is loaded. Therefore, in production, for example, a 2000 Hz, KrF excimer laser may operate at a duty factor of about 30 percent. The operation is 24 hours per day, seven days per week, 52 weeks per year. A laser operating at 2000 Hz xe2x80x9caround the clockxe2x80x9d at a 30 percent duty factor will accumulate more than 1.5 billion pulses per month. Any disruption of production can be extremely expensive. For these reasons, prior art excimer lasers designed for the lithography industry are modular so that maintenance down time is minimized.
Maintaining high quality of the laser beam produced by these lasers is very important because the lithography systems in which these laser light sources are used are currently required to produce integrated circuits with features smaller than 0.25 microns and feature sizes get smaller each year. Laser beam specifications limit the variation in individual pulse energy, the variation of the integrated energy of series of pulses, the variation of the laser wavelength and the magnitude of the bandwidth of the laser beam.
Typical operation of electric discharge lasers such as that depicted in FIG. 1 causes electrode erosion. Erosion of these electrodes affects the shape of the discharge which in turn affects the quality of the output beam as well as the laser efficiency. Electrode designs have been proposed which are intended to minimize the effects of erosion by providing on the electrode a protruding part having the same width as the discharge. Some examples are described in Japanese Patent No. 2631607. These designs, however, produce adverse effects on gas flow. In these gas discharge lasers, it is very important that virtually all effects of each pulse be blown out of the discharge region prior to the next pulse.
What is needed is a gas discharge laser having erodable electrodes which do not adversely affect gas flow and can withstand many billions of pulses without adversely affecting laser beam quality.
The present invention provides a gas discharge laser having a laser chamber with two elongated erodable electrode elements, each having an erodable section and an electrode support configured to minimize discharge region laser gas turbulence and with the electrode elements being configured to permit gradual erosion over more than 8 billion pulses without causing substantial changes in the shape of electrical discharges between the electrode elements. A pulse power system provides electrical pulses of at least 2 J at rates of at least 1 KHz. A blower circulates laser gas between the electrodes at speeds of at least 10 m/s and a heat exchanger is provided to remove heat produced by the blower and the discharges.